Photoconduction in an electrophotographic photoreceptor consists of the following two steps:
(1) generation of electric charges by exposure, and
(2) transport of the generated electric charges.
A selenium photoconductor may be mentioned as a typical example in which the steps (1) and (2) are achieved using a single substance. On the other hand, the combination of amorphous selenium and polyvinylcarbazole is familiar example in which the two steps (1) and (2) are effected separately by different types of substances. The latter combination has the advantage that a wider range of selection for photoconductive materials is possible, thus helping achieve better electrophotographic characteristics, such as higher photosensitivity and receptive potential, as well as greater ease in manufacturing the photoreceptor layer.
Inorganic substances, such as selenium, cadmium sulfide and zinc oxide, have long been used as photoconductive material in electrophotographic photoreceptors.
As disclosed in U.S. Pat. No. 2,297,691, the electrophotographic process employs a photoconductive material comprising a base material and a substance coated thereupon which is electrically insulated in the dark and whose electric resistance changes depending on the amount of light received during exposure. The conductive material is uniformly surface charged after being allowed to stand in the dark for an appropriate time, and then exposed for image formation in such a pattern that the surface charge at each section of the photoconductive material is reduced in accordance with the relative energy at that image section. The pattern of electric charges thus left on the surface of photoconductive layer (electrophotographic photoreceptive layer), that is, static latent images, are then brought into contact with a charge-detecting and developing substance ("toner") for development of visible images.
The toner, whether it is dispersed in an electrically insulating liquid or contained in a dry carrier, can be attached to the electrophotographic photosensitive layer in accordance with the pattern of static charges. The toner thus attached can be fixed by known methods, such as application of heat, pressure and solvent vapor. Alternatively the static latent images may be transported to a second substrate, such as a paper and a film. It is also possible to transport the static latent images to a second substrate and develop them there. Electrophotography is the process of forming images by these methods.
The basic requirements for photoreceptors used in this process include: (1) the capability of being electrically charged to an appropriate potential in the dark, (2) little leakage of electric charges in the dark, and (3) rapid leakage of electric charges upon irradiation of light.
The inorganic substances mentioned above have various advantages, but it is also true that they also have a number of disadvantages. Selenium, which is now used extensively, fully satisfies the above-mentioned requirements (1) through (3). On the other hand, the cost for producing photoreceptors is high because of the critical manufacturing conditions involved. It is difficult to shape this substance into a belt form because of a lack of flexibility, and its high sensitivity to thermal and mechanical shock requires special care in handling. Cadmium sulfide and zinc oxide are used in a dispersed form using a resin as a binder, but poor mechanical properties, such as surface smoothness, hardness, tensile strength and abrasion resistance, prove an obstacle against its repetitive use.
To avoid these difficulties associated with inorganic substances, various photoreceptors using organic substances have recently been proposed, some of which have been practically used. These include photoreceptors comprising poly-N-vinylcarbazole and 2,4,7-trinitrofluoren-9-one (as disclosed in U.S. Pat. No. 3,484,237); poly-N-vinylcarbazole sensitized with pyrilium salt dyes (as disclosed in Japanese Patent Publication No. 25658/73); photoreceptors using an organic pigment as main component (as disclosed in Japanese Patent Application (OPI) No. 37543/72 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application")); and photoreceptors using as a main component an eutectic complex of a dye and a resin (as disclosed in Japanese Patent Application (OPI) No. 10735/72).
These organic photoreceptors have better mechanical properties and flexibility in comparison with the above-described inorganic photoreceptors to a greater or lesser extent, but they do not fully satisfy the requirements because of low sensitivity or unsuitability for repetitive use.